The present invention relates to a system for controlling the distribution of a driving force between front and rear wheel pairs of a four wheel drive vehicle by varying torque transmitted from an engine to either of the front and rear wheel pairs with a clutch.
A four wheel drive vehicle exhibits a superior ability on a road surface of a low coefficient of friction by dispersing its driving force among front and rear wheels. However, a 4WD vehicle has a disadvantage of a tight corner brake phenomenon which appears during a turn of the vehicle on a road surface of a high coefficient of friction because of a rigid connection between the front and rear wheels.
Japanese Utility Model provisional publication Nos. 58-100132 and 58-139226 disclose four wheel drive vehicles having a control system for controlling the changeover between a two wheel drive mode and a four wheel drive mode in accordance with a vehicle's running condition.
A four wheel drive vehicle shown in the document No. 58-100132 is arranged to prevent the driving wheels from spinning by changing over its drive system from the two wheel drive mode to the four wheel drive mode when a rain sensor senses a decrease of the friction coefficient of a road surface due to rain. A four wheel drive vehicle shown in the document No. 58-139226 is arranged to prevent the tight corner braking by changing over its drive system from the four wheel drive mode to the two wheel drive mode when an oil pressure sensor senses a turn of the vehicle of a great steering amount by sensing an oil pressure of a hydraulic power steering system.
However, it is not sufficient to prevent only the wheel spin on the tight corner brake, but it is necessary to prevent both of them. If the teachings of both documents were combined, it would require various sensors such as rain sensor, oil pressure switch and vehicle speed sensor so that the manufacturing cost would be increased and the construction would be complicated.
Furthermore, the four wheel drive vehicles of both documents are disadvantageous in that the vehicle steering stability is impaired by an abrupt change of the steering characteristics due to an abrupt change between the two and four wheel drive modes.